1. Field of the Invention
The present invention relates to ladder filters and branching filters used, particularly for communication apparatuses, such as portable telephones, and to communication apparatuses.
2. Description of the Related Art
In recent years, remarkable technological progress has been made in reducing the size and weight of communication apparatuses, such as portable telephones. In order to achieve such technological progress, development of composite components having a plurality of functions, as well as reduction and miniaturization of each component, has been achieved. In particular, improved performance of surface acoustic wave filters is required.
Ladder-type surface acoustic wave filters including surface acoustic wave resonators alternately disposed on a series arm and a parallel arm are one of such surface acoustic wave filters.
As shown in FIG. 23, a well-known ladder-type surface acoustic wave filter 100 includes two series resonators S111 and S112 and three parallel resonators P111, P112, and P113. Also, in the ladder-type surface acoustic wave filter 100, the anti-resonant frequency fpa100 of the parallel resonators P111, P112, and P113 is set so as to be substantially equal to the resonant frequency fsr100 of the series resonators S111 and S112. Since conditions for setting the anti-resonant frequency fpa100 of the parallel resonators P111, P112, and P113 and the resonant frequency fsr100 of the series resonators S111 and S112 are limited in the ladder-type surface acoustic wave filter 100, there is a problem in that it is difficult to increase the pass band width.
Other examples of ladder-type surface acoustic wave filters are disclosed in Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-126212, Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-303698, and Patent Document 3: Japanese Unexamined Patent Application Publication No. 9-167937.
According to a ladder-type surface acoustic wave filter disclosed in Patent Document 1, the ladder-type surface acoustic wave filter 100 is arranged such that the resonant frequency fsr100 of series resonators and the anti-resonant frequency fpa100 of parallel resonators are set so as to satisfy the condition fsr100>fpa100. This arrangement increases the frequency interval between the series resonators and the parallel resonators. Thus, the pass band width of the ladder-type surface acoustic wave filter is increased.
With the arrangement of Patent Document 1, however, the frequency interval between the series resonators and the parallel resonators is increased, a ripple is generated in the pass band and the voltage standing wave ratio (VSWR) is increased. This limits the increase of the band width.
According to a ladder-type surface acoustic wave filter disclosed in Patent Document 2, the ladder-type surface acoustic wave filter 100 is arranged such that the resonant frequency of one series resonator is different from the resonant frequency of the other series resonator. Thus, the power resistance in the ladder-type surface acoustic wave filter is improved.
With the arrangement of Patent Document 2, however, the combined resonant frequency when the impedances of the two series resonators are combined must be substantially equal to the anti-resonant frequency of parallel resonators. This arrangement provides a pass band width between the resonant frequency of the parallel resonators and the lower one of the anti-resonant frequencies of the two series resonators. With this arrangement, the band width is reduced in accordance with an increase in the frequency interval between the resonant frequencies of the two series resonators.
As shown in FIG. 24, a ladder-type surface acoustic wave filter 200 disclosed in Patent Document 3 includes three series resonators S211, S212, and S213 and two parallel resonators P211 and P212. In the ladder-type surface acoustic wave filter 200, the resonant frequency of the series resonators S211, S212, and S213 is set so as to be substantially equal to the anti-resonant frequency of the parallel resonators P211 and P212. An inductor L211 is added in parallel to the series resonator S213. The anti-resonant frequency of the series resonator S213 is shifted towards higher frequencies due to the inductor L211, which increases the attenuation at frequencies higher than the pass band of the ladder-type surface acoustic wave filter.
With the arrangement of Patent Document 3, however, the band width of the ladder-type surface acoustic wave filter cannot be increased.
As described above, due to the various problems with conventional ladder-type surface acoustic wave filters, it is difficult to increase the pass band width of the ladder-type surface acoustic wave filters with the known arrangements.
The problems described above are also produced in a ladder filter including piezoelectric thin-film resonators including a silicon (Si) substrate and a vibrating portion. The Si substrate is provided with an opening or a recess. The vibrating portion is configured such that the top and bottom surfaces of a piezoelectric thin film composed of at least one or more layers made of, for example, zinc oxide (ZnO) and aluminum nitride (AlN), and arranged above the opening or the recess are sandwiched by at least a pair of upper and lower electrodes which face each other.